The present invention relates to manufacturing articles by molding them from fluent materials, and more particularly, to an apparatus and method which is particularly adapted to minimize or eliminate problems commonly occurring in the molding of rubber and other materials, and particularly in so-called insert molding.
One field wherein insert molding techniques are commonly employed is the oil seal field. Perhaps the most common type of oil and grease seal made today is one wherein a seal body is provided which has one or more sealing lips made from a natural or synthetic elastomer which is bonded to, or which surrounds or encloses at least a part of a stiff casing or other structural member.
Typically, oil seals made to act radially on a shaft comprise an exterior annular metal casing or cup of L-shaped or other suitable cross section, made by a simple stamping operation and having bonded to the inner periphery thereof an annular rubber body having one or more sealing lips thereon.
The casing, stamping or stiffener is used to achieve rigid mounting of the seal, and the elastomeric seal body is designed to provide fluid retention by engagement in fluid-tight relation of an associated shaft, and also to provide tolerance of eccentric motion, which to a greater or less degree, occurs in shafts of all kinds. The expression "insert molding" applies to a molding method wherein a metal or other stiff "insert" is disposed entirely within or projects partially into the cavity in which the seal body is formed by molding. The invention is useful in molding ordinary rubber or plastic parts as well as in insert molding.
Today, most fluid seals are made by so-called compression molding, by which is meant a method wherein a selected piece of rubber, appropriately compounded and shaped, is forced by the simple compressing or closing action of mold closure into a cavity which is completely defined only when the mold is fully closed.
A large number of seals are also made by so-called injection molding, wherein the mold is fully closed over the insert before any rubber enters the mold, and wherein rubber is thereafter injected through one or more small orifices or sprues into the cavity, usually by a screw-type injector.
A certain number of seals are also made by "transfer molding," a process combining certain features of both compression molding and injection molding.
In general, compression molding is desirable because of economy and relative simplicity, especially where large numbers of parts are to be made in multiple-cavity molds. This is because a single press may accommodate a large number of molding cavities, and accordingly, one "heat" or cycle of a molding press may produce 24, 36, 48 or other large number of parts, for example.
When the inherent economies of compression molding are most apt to be realized, however, certain problems invariably occur which create problems of quality control. This is especially true where the article in question, such as an oil seal, is finish molded to its exact size rather than being molded to an approximate size and later trimmed to its exact size.
Specifically, a very common problem with compression molding is securing precise filling of each individual mold cavity, together with insuring that the contents of each mold cavity are maintained under a suitably high pressure.
It will be understood that if a mold cavity is not completely filled, the article will not have the desired shape, and will be characterized by voids, pock marks, blisters, and a generally irregular shape. If sufficient pressure is not maintained in the mold, a porous finished product may result, because of the undesirable inclusion into the molded product of gases generated during curing of the rubber. On the other hand, if an excess of material is provided, mold closing forces will cause the rubber material, which is substantially incompressible when totally confined, to leak or "flash" from the mold cavity, usually along the insert. This flash is undesirable, because it must either be removed from the finished product by a separate operation, or left on the finished product, which is also undesirable for functional or esthetic reasons, or otherwise.
Inasmuch as the size of the individual piece or rubber from which the molded part is made, sometimes referred to as the "prep," is not easy to control accurately within precise limits, especially without undue expense, most compression molded parts are either characterized by certain amount of flash, or the molders have resigned themselves to eliminating flash by performing operations subsequent to molding.
Certain attempts have been made to overcome these problems, and some of these proposals have been successful, at least in certain respects, to a greater or less degree. However, one of the most common methods involves actually reforming the stamping in the mold after the stamping has been produced, with resultant wear and shock on the mold. Moreover, where the seal body is joined to a flange of the stamping which is offset, the lip design must be relatively reduced in axial extent, which provides a seal of reduced flexibility, and which presents other disadvantages.
In view of the foregoing and other characteristics of prior art molds and methods, particularly those used for insert molding, as in the seal making art, it is an object of the present invention to provide an improved apparatus and methods for use in molding.
Another object is to provide an apparatus which is particularly adapted to retain the advantages of compression molding without possessing certain of the disadvantages usually associated therewith.
Another object of the invention is to provide an apparatus adapted to simplify compression molding and to improve the quality of the products made thereby.
Yet another object is to provide an apparatus capable of producing satisfactory parts even where there is a significant variation in the amount of raw material supplied for molding in the cavity.
A still further object is to provide an apparatus which will apply a predetermined force to fluent material during flow and curing thereof, but which will not exceed such force, even when the mold is fully closed.
Another object is the provision of a mold operable by a single press or platen but which is characterized by double-acting or sequential operation.
Another object is to provide a mold which makes possible accurate molding of parts, including insert molding, in a multi-cavity operation, even where there is a variation in the weight or volume of raw material pieces supplied to the mold.
Another object is the provision of an insert molding apparatus which includes means for positioning and holding the insert prior to molding, means for establishing a flash barrier along a part of the insert, means for defining at least a part of the cavity, and means movable relative to the means establishing a flash barrier for moving the molding material from an initial position spaced apart from the molding cavity through a transfer or tear trim area and into the molding cavity.
A still further object is to provide an apparatus having a stationary mold part or member partially defining a molding cavity and two additional mold parts, both of which are movable relative to the fixed part and to each other, and adapted for sequential operation.
Another object is the provision of a mold having relatively movable parts, with such mold including means for forcibly but resiliently urging such parts into a predetermined position for defining a molding cavity and insuring complete filling thereof by fluent moldable material without creating flash or other leakage and also having means for accommodating material in excess of that used to mold the part or article in question.
A still further object is to provide a method of compression molding which includes positioning and holding an insert in a desired position, establishing one or more flash barriers and establishing the contours of the molding cavity, thereafter filling such cavity with fluent, moldable material, and thereafter maintaining a predetermined force on said material during at least a portion of the curing time of the material, all of such steps occurring in sequence within the mold assembly upon application thereto of a single force supplied by a press or the like.
Another object is to provide a mold apparatus having a fixed mold core portion with surfaces thereon defining a part of a molding cavity, a second mold part movable relative to the first and having closing spring means associated therewith, a third mold part movable relative to the first and second mold parts and also having a closing spring associated therewith, and means for engaging both of said springs for moving said second and third mold parts along a common axis to a closed position in a predetermined sequence.
Still another object is to provide a mold apparatus of the type referred to herein which includes means for separating the excess or scrap material from the part as the mold is opened to remove the finished part therefrom.
Another object is to provide a mold of this type which further includes a resiliently operated hold-down device for positioning inserts or the like.
The foregoing and other objects are achieved in practice by providing a mold apparatus having a first, fixed mold member and second and third mold members movable along a given axis relative to the first member and relative to each other, resilient means associated with each of the second and third parts, with the resilient means being adapted to be engaged by one or more parts for effecting a movement sequence so as to define a molding cavity between the first and second members and subsequently fill the cavity by applying a limited force to the third member to move molding material from the area in which it is received to the mold cavity.
The manner in which the foregoing and other objects and advantages of the invention are achieved in practice will be more clearly apparent when reference is made to the detailed description of certain preferred embodiments of the invention set forth by way of example, and illustrated in the accompanying drawings, in which like reference characters indicated corresponding parts throughout.